The present invention relates to a method and a device for the detection of the garbling of pulses received by a secondary radar. It can be applied notably to secondary radars receiving pulse responses from several aircraft at a time.
With air traffic becoming increasingly dense, the pulses sent out by the transponders of aircraft towards secondary radars on the ground get jumbled, giving rise to phenomena of pulse-garbling. These garbling phenomena induce false codes in the reception and processing circuits of the secondary radars, which prevents the identifying of the aircraft or of their positions for example, and thereby leads to serious consequences for air traffic safety.
The presence of a pulse of a secondary response sent out by an aircraft is generally detected by means of a quantified signal referenced Q.SIGMA., the high state of which indicates the passing to mid-amplitude of the signal received by the sum channel, referenced .SIGMA., of the antenna of the secondary radar. The processing done by the secondary radar makes use of this type of information only, firstly in order to detect a response, especially the two known and standardized pulses which are separated by a period of 20.3 .mu.s and sandwich the response and, secondly, to detect the code of the response having the form of a succession of pulses. Consequently, the entire secondary processing operation makes use only of the presence of high frequency power contained in pulses with a standardized duration equal to 450 ns separated from one another by a standardized pitch that is a multiple of 1.45 .mu.s. Each pulse conveys a binary information element. Consequently, if its power exceeds a certain threshold which is a function of the reference pulses F1 or F2, this information element will be equal, for example, to 1. If not, it will be equal to 0. The power sent out by the transponders of the aircraft is generally transposed, at reception, into the range of the so-called intermediate frequencies, typically of the order of 60 MHz, and is then detected through logarithmic limiter amplifiers aimed notably at absorbing the wide dynamic range of power received and at preventing, for example, the saturation of the processing circuits.
In the event of garbling between two pulses received by a secondary radar, the prior art methods of analysis, notably the systems used to analyze the power of the signal Q.SIGMA., do not reveal the existence of two intermingled (hence garbled) pulses when the difference in power between these pulses is smaller than 6 dB for example, i.e. when the power of one pulse is not at least equal to twice the power of the other pulse. Errors of detection are then made as regards the duration and position of the garbled pulses.